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Partial lunar eclipse this SaturdayBY DR EMILY BALDWIN
ASTRONOMY NOW

Posted: August 12, 2008

Following hot on the heels of the solar eclipse at the start of the month is a partial lunar eclipse, gracing our evening skies on Saturday 16 August.

The Moon often appears to turn red during a lunar eclipse, because red light is refracted through the Earth's atmosphere while blue light is peferentially scattered.

A partial lunar eclipse occurs when the Earth, Sun and Moon are almost exactly in line, with the Earth sandwiched between the Sun and Moon. The eclipse occurs as the full moon moves partly into the shadow cast by the Earth. The Earth’s shadow can be divided into two distinctive parts: the umbra and penumbra. Within the umbra there is no direct solar illumination, but because of the Sun’s large angular size, this illumination is only partially blocked in the outer portion of the Earth’s shadow, the penumbra.

A partial lunar eclipse only occurs when a portion of the Moon enters the umbra, and in Saturday’s event, over 80 percent of the Moon’s visible side will be covered by the umbra, with the remainder in the penumbra. Although the Moon travels to a similar position every month, the tilt of the lunar orbit means that it normally passes above or below the terrestrial shadow so a full moon is seen but no eclipse takes place.

The eclipse on Saturday begins at 1923 BST when the Moon enters the penumbra. Soon after the Moon will have a slight yellowish hue. From London the eclipsed Moon will be visible after it rises at 2011 BST while observers in Glasgow will see it from 2040 BST. The Moon enters the umbra at 2036 BST and the greatest eclipse occurs at 2210 BST. Stronger scattering of blue light means that the light that reaches the lunar surface has a reddish hue, so observers on Earth will see a Moon that is partly light and partly dark, with hints of colour. The Moon leaves the umbra at 2344 BST and finally the eclipse finishes when it exits the penumbra at 0057 BST on 17 August.

This eclipse will be best seen from most of Africa, Eastern Europe, central Asia, India and the Middle East. From the UK, the Moon will be fairly low in the sky throughout the event, and unlike an eclipse of the Sun, the whole event is quite safe to watch with the naked eye or through telescopes and binoculars, and needs no special equipment.

As usual, we would love to see any images that you have taken. Email them, along with observing details, to eclipse2008<at>astronomynow.com with ‘eclipse’ as the message title.

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